Conductive shield for external press type electron discharge tubes



May 13, 1947. N. P. GOWELL 2,420,311.

CONDUCTIVE SHIELD FOR EXTERNAL PRESS TYPE ELECTRON DISCHARGE TUBES Filed Feb. 10, 1945 Patented May 13, 1947 CONDUCTIVE SHIELD FOR EXTERNAL PRE S S TYPE TUBES ELECTRON DISCHARGE Niles P. Gowell, Watertown, Mass, assignor to Raytheon Manufacturing Company, Newton, Mass, a corporation of Delaware Application February 10, 1945, Serial No. 57 7,169

6 Claims.

The present invention relates to electron discharge tubes, and more particularly to those of the fiat bulb type, such as the small space discharge tubes used in hearing aids, pocket radios or other portable devices intended to be carried on the person.

In these small tubes, the various elements are of necessity close together which tends to increase the capacity between these elements. This is particularly true of the lead-in wires which are sealed through a fiat press in a single line and closely spaced.

It is one of the objects of the present invention to reduce the capacity between certain tube elements and more particularly between the leadin wires.

The aforesaid object and such other aims and objects of the present invention as may hereinafter appear will be best understood from the following description, taken in connection with the accompanying drawing, of a space discharge tube embodying one illustrative embodiment of the present invention.

In the drawing:

Fig. 1 is a front elevation, on an enlarged scale, of a tube incorporating one illustrative embodiment of the present invention;

Fig. 2 is a side elevation of the tube shown in Fig.

Fig. 3 is a longitudinal section of a fragment of the wall of the bulb, on an enlarged scale, showing the glass wall, the conductive shield and the protective coating, it being understood that said conductive shield and protective coating may extend over the whole outer surface of the bulb and down over the lateral surface of the press to the lower edge of the latter from which the lead-in wires project; and

Fig. l shows diagrammatically an electrode assembly, by way of illustration, that may be used.

The drawing shows a small space discharge tube of the general type above referred to having a relatively flat bulb 2 hermetically sealed and terminating at its base in a relatively flat press 4. Said bulb 2 contains a conventional electrode assembly diagrammatically represented in Fig. 4. In the illustrative embodiment of the invention shown in the drawing, said electrode assembly comprises a filament cathode 6, an anode 8, a control grid Ill adjacent said cathode, a suppressor grid l2 adjacent said anode and a screen grid [4 between said control grid and said suppressor grid. The lead-in conductors for the electrode assembly are indicated at IE.

I have discovered that if an electrically conductive shield i8 be applied to the outer surface of said bulb 2 and to the lateral surface of the press, leaving the narrow end surface of the press from which the lead-in wires l6 issue free of said shield so as not to short-circuit said lead-in wires, the capacity between the plate and the control grid will be reduced to a marked degree. This is particularly true of the capacity between the lead-in wires, reduction of which also reduces the grid-plate capacity.

If desirable the conductive shield [8 may be applied only to a portion of the tube, for example only to the lateral wall of the press.

The present invention contemplates the use of any suitable electrically-conductive material for the shield. Certain metals, such as copper and especially silver, are very satisfactory. The application of the material for the shield may be made in any suitable form or way, as in liquid form, for example by dipping, spraying or brushing. The application may also be made in the form of a quasi emulsive; that is to say, with the electrically conductive material in a very finely divided state and in suspension in a suitable liquid vehicle. Carbon may be applied in this Way as an electrically conductive shield, water or oil serving as the vehicle. Silver may also be applied in this way, in which case the du Pont silver coating material No. 4503 which contains the finely divided silver in suspension may be used.

When the shield has been applied, it may be covered by any suitable protective coating I9. I preferably use a protective coating comprising essentially Vinylite resin, Pent-Acetate (acetates of isomeric alcohols) and acetone. The cold bulb coated with the dried conductive shield is dipped in the protective coating mixture, and then heated in an oven for about fifteen minutes at 0., whereupon it is removed from the oven and allowed to cool. In use the conductive shield should be grounded, as shown at 20.

Owing to flatness of the bulb, its surfaces are brought close to the tube elements so that the conductive shield applied to these surfaces is also brought close to said elements and is thus effective to reduce the capacity between them. In the case of the press, only a thin wall of glass separates the shield from the lead-in wires sealed through said press, so that the effect of said shield on said lead-in wires is still greater and results in a very material reduction of the capacity between said lead-in wires and consequently a material reduction in the grid-plate capacity.

I am aware that the present invention can be embodied in other specific forms without depart- 3 ing from the spirit or essential attributes thereof, and I therefore desire th present description to be considered in all respects as illustrative and not restrictive, reference being had to the appended claims rather than to the aforesaid description to indicate the scope of the invention.

What is claimed is:

1. An electron discharge tube comprising a bulb and an exterior flat press, and an electrically conductive shield applied to the exterior lateral surface of said fiat press.

2. An electron discharge tube comprising a bulb and an exterior flat press, and a, shield of metallic silver applied to the exterior lateral surface of said press.

3. An electron discharge tube comprising a bulb and an exterior flat press, and an electrically conductive shield comprising minute particles of electrically conductive material applied to the exterior surface of said flat press.

4. An electron discharge tube comprising a bulb and an exterior flat press, and an electrically conductive shield comprising minute particles of carbon applied to the exterior lateral surface of said press.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,350,003 West May 30, 1944 2,013,095 Friend Sept. 3, 1935 1,802,371 Bullivant et al Apr. 28, 1931 1,909,769 Krahl May 16, 1933 

